1. Field of the Invention
The present invention is directed, in general, to wireless communication systems and, more specifically, to a Single-Carrier Frequency Division Multiple Access (SC-FDMA) communication system and is further considered in the development of the 3rd Generation Partnership Project (3GPP) Evolved Universal Terrestrial Radio Access (E-UTRA) long term evolution (LTE).
2. Description of the Art
In particular, the present invention considers partitioning resources allocated to the transmissions of control signals and data signals in a SC-FDMA communication system. The invention assumes the UpLink (UL) communication corresponding to signal transmissions from mobile User Equipments (UEs) to a serving base station (or Node B). A UE, also commonly referred to as terminal or mobile station, may be fixed or mobile and may be a wireless device, a cellular phone, a personal computer device, a wireless modem card, etc. A Node B is generally a fixed station and may also be called a Base Transceiver System (BTS), an access point, or some other terminology.
Several types of signals need to be supported for the proper functionality of the communication system. In addition to data signals, which convey the information content of the communication, control signals also need to be transmitted from the UEs to their serving Node B in the UL and from the serving Node B to the UEs in the DownLink (DL) in order to enable the proper transmission of data signals. The DL refers to the communication from the Node B to UEs. These control signals are subsequently described in detail with the focus being on the UL.
The UEs are assumed to transmit data signals (or data packets) through the Physical Uplink Shared CHannel (PUSCH). The PUSCH can be shared during the same time period by multiple UEs with each UE using a different part of the operating BandWidth (BW), as illustrated in FIG. 1, in order to avoid mutual interference (Frequency Domain Multiplexing (FDM)). UE1 110 transmits over BW 120 while UE2 130, UE3 150, and UE4 170, transmit over BW 140, BW 160, and BW 180, respectively. An exception is the use of Spatial Division Multiple Access (SDMA) methods, where multiple UEs may share the same RBs over the same sub-frame for their PUSCH data packet transmissions.
The Node B is assumed to transmit data signals (or data packets) to UEs through the Physical Downlink Shared CHannel (PDSCH). Similarly to the PUSCH, the PDSCH can be shared during the same time period by multiple UEs through FDM.
PUSCH and PDSCH transmissions can be scheduled by the Node B through a UL or a DL scheduling assignment, respectively, using the Physical Downlink Control CHannel (PDCCH) or they can be preconfigured to occur periodically (persistent scheduling of PUSCH or PDSCH transmissions). Using the PDCCH, a data signal transmission in the PUSCH or the PDSCH may generally occur at any sub-frame decided by the Node B scheduler. Accordingly, the scheduling of such transmissions is referred to as dynamic.
To avoid excessive PDCCH overhead, some PUSCH and PDSCH transmissions may be configured to occur periodically at predetermined parts of the operating bandwidth. Such scheduling is referred to as persistent. FIG. 2 illustrates the concept of persistent scheduling where an initial packet transmission 210 occurs periodically every assignment interval 220. Persistent scheduling is typically used for communication services having relatively small bandwidth requirements per transmission period but need to be provided for many UEs making dynamic scheduling through the PDCCH inefficient due to the associated overhead introduced in the DL of the communication system. One typical example of such services is Voice over Internet Protocol (VoIP).
In response to the PUSCH and PDSCH transmissions, positive or negative acknowledgement signals, ACK or NAK respectively, are assumed to be transmitted to or from the UEs, respectively. As the invention considers the UL of the communication system, the focus will be on the ACK/NAK signals transmitted by UEs in response to a PDSCH transmission. ACK/NAK signaling is required for use of Hybrid-Automatic Repeat reQuest (HARQ), where a data packet is retransmitted upon the reception of a NAK and a new data packet it transmitted upon the reception of an ACK.
Because the PDSCH scheduling of a UE in the DL can be dynamic or persistent, the transmission of ACK/NAK signals from the UE is correspondingly dynamic or persistent. In the latter case, similarly to the PDSCH transmission, the ACK/NAK transmission from the UE is periodic.
In addition to periodic and dynamic transmission of ACK/NAK signals, other control signals may be periodically transmitted by UEs. One example of such a control signal is the Channel Quality Indication (CQI). The CQI is assumed to be sent periodically to inform the serving Node B of the channel conditions, which can be represented by the Signal-to-Noise and Interference Ratio (SINR) the UE experiences in the DL. Additional periodic transmissions of control signals other than CQI or ACK/NAK may also exist.
Therefore, the UL of the communication system is assumed to support dynamic and persistent PUSCH transmissions, ACK/NAK transmissions due to dynamic and persistent PDSCH transmissions, CQI transmissions, and possibly other control signaling. The transmissions of CQI, persistent PUSCH, and ACK/NAK due to persistent PDSCH are assumed to be periodic until deactivated by the serving Node B or until the corresponding configured transmission period expires. The ACK/NAK and CQI signals will be jointly referred to as the Physical Uplink Control CHannel (PUCCH). Other control signals may also be periodically transmitted in the PUCCH.
The PUSCH transmissions are assumed to occur over a Transmission Time Interval (TTI) corresponding to a sub-frame. FIG. 3 illustrates a block diagram of the sub-frame structure 310 assumed in the exemplary embodiment of the disclosed invention. The sub-frame includes of two slots. Each slot 320 further includes seven symbols and each symbol 330 further includes a Cyclic Prefix (CP) for mitigating interference due to channel propagation effects. The signal transmission in the two slots may or may not be in the same part of the operating bandwidth.
In an exemplary sub-frame structure of FIG. 3, the middle symbol in each slot carries the transmission of Reference Signals (RS) 340, also known as pilot signals, which are used for several purposes including for providing channel estimation to allow coherent demodulation of the received signal. The number of symbols with RS transmission in the UL sub-frame may be different among the PUSCH, the PUCCH with ACK/NAK transmission, and the PUCCH with CQI transmission. For example, the middle three symbols in each slot may be used for RS transmission in case of ACK/NAK PUCCH transmissions (the remaining symbols are used for ACK/NAK transmission) while the second and sixth symbols in each slot may be used for RS transmission in case of CQI PUCCH transmissions (the remaining symbols are used for CQI transmission). This is also illustrated in FIG. 9, FIG. 10, and FIG. 11, which will be described later herein.
The transmission bandwidth is assumed to comprise of frequency resource units, which will be referred to as Resource Blocks (RBs). The exemplary embodiment assumes that each RB includes 12 SC-FDMA sub-carriers and UEs are assumed to be allocated a multiple N of consecutive RBs 350 for PUSCH transmission and 1 RB for PUCCH transmission. Nevertheless, the above values are only illustrative and not restrictive to the invention.
Although not material to the disclosed invention, an exemplary block diagram of the transmitter structure for the PUSCH is illustrated in FIG. 4. If a UE has both data and control (ACK/NAK, CQI, etc.) bits to transmit in the same PUSCH sub-frame, then, in order to transmit the ACK/NAK, certain data bits (such as, for example, the parity bits in the case of turbo coding) may be punctured and replaced by the ACK/NAK bits. Simultaneous PUSCH and PUCCH transmission by a UE is thus avoided and the single-carrier property is preserved. Coded CQI bits 405 (if they exist) and coded data bits 410 are multiplexed 420. If ACK/NAK bits also need to be transmitted in the PUSCH, data bits (or possibly CQI bits) are punctured to accommodate ACK/NAK bits 430. The Discrete Fourier Transform (DFT) of the combined data bits and control bits is then obtained 440, the sub-carriers 450 corresponding to the assigned transmission bandwidth are selected 455, the Inverse Fast Fourier Transform (IFFT) is performed 460 and finally the Cyclic Prefix (CP) 470 and filtering 480 are applied to the transmitted signal 490.
Zero padding is assumed to be inserted by a reference UE in sub-carriers used by another UE and in guard sub-carriers (not shown). Moreover, for brevity, additional transmitter circuitry such as digital-to-analog converter, analog filters, amplifiers, and transmitter antennas as they are known in the art, are not illustrated in FIG. 4. Similarly, the encoding process for the data bits and the CQI bits as well as the modulation process for all transmitted bits are well known in the art and are omitted for brevity.
At the receiver, the inverse (complementary) transmitter functions are performed. This is conceptually illustrated in FIG. 5 where the reverse operations of those in FIG. 4 apply. As it is known in the art (not shown for brevity), an antenna receives the Radio-Frequency (RF) analog signal and after further processing units (such as filters, amplifiers, frequency down-converters, and analog-to-digital converters) the digital received signal 510 passes through a time windowing unit 520 and the CP is removed 530. Subsequently, the receiver unit applies an FFT 540, selects 545 the sub-carriers 550 used by the transmitter, applies an Inverse DFT (IDFT) 560, extracts the ACK/NAK bits and places respective erasures for the data bits 570, and de-multiplexes 580 the CQI bits 590 and data bits 595. As for the transmitter, well known in the art receiver functionalities such as channel estimation, demodulation, and decoding are not shown for brevity and they are not material to the invention.
Also without being material to the disclosed invention, a block diagram of the PUCCH (ACK/NAK, CQI) transmission structure is illustrated in FIG. 6. The transmission is assumed to be through the modulation of Constant Amplitude Zero Autocorrelation (CAZAC)-based sequences 610. Similarly, the RS transmission is assumed to be through non-modulated CAZAC-based sequences 610. The sub-carriers corresponding to the assigned transmission bandwidth are selected 620 and the sequence elements are mapped on the selected PUCCH sub-carriers 630. The Inverse Fast Fourier Transform (IFFT) is performed 640, the output is then cyclically shifted in the time domain 650, and finally the Cyclic Prefix (CP) 660 and filtering 670 are applied to the transmitted signal 680. With respect to the PUSCH transmitter structure in FIG. 4, the main difference is the absence of a DFT block (because, although not required, the CAZAC-based sequence is assumed to be directly mapped in the frequency domain to avoid the DFT operation) and the application of the cyclic shift 650. In addition, Walsh covering may apply to the ACK/NAK, RS, and possibly the CQI signals across the corresponding symbols in the sub-frame (FIG. 3).
The reverse functions are performed for the reception of the CAZAC-based sequence as illustrated in FIG. 7. The received signal 710 passes through a time windowing unit 720 and the CP is removed 730. Subsequently, the cyclic shift is restored 740, an FFT 750 is applied, the sub-carriers 760 used by the transmitter are selected 765, correlation with the replica 770 of the CAZAC-based sequence is applied 780 and the output 790 is obtained. The output can be passed to a channel estimation unit, such as a time-frequency interpolator, in case of an RS, or can be used for detecting the transmitted information, in case the CAZAC-based sequence is modulated by ACK/NAK or CQI information bits.
An example of CAZAC-based sequences is given by the following Equation (1):
                                          c            k                    ⁡                      (            n            )                          =                              exp            ⁡                          [                                                                    j                    ⁢                                                                                  ⁢                    2                    ⁢                                                                                  ⁢                    π                    ⁢                                                                                  ⁢                    k                                    L                                ⁢                                  (                                      n                    +                                          n                      ⁢                                                                        n                          +                          1                                                2                                                                              )                                            ]                                .                                    (        1        )            
In Equation (1), L is the length of the CAZAC sequence, n is the index of a particular element of the sequence n={0, 1, 2 . . . , L−1}, and finally, k is the index of the sequence itself. For a given length L, there are L−1 distinct sequences, provided that L is prime. Therefore, the entire family of sequences is defined as k ranges in {1, 2 . . . , L−1}. However, the CAZAC sequences used for PUCCH signaling need not be generated using the exact above expression as it is further discussed below.
For CAZAC sequences of prime length L, the number of sequences is L−1. As the RBs are assumed to include an even number of sub-carriers, with 1 RB includes 12 sub-carriers, the sequences used to transmit the ACK/NAK and RS can be generated, in the frequency or time domain, by either truncating a longer prime length (such as length 13) CAZAC sequence or by extending a shorter prime length (such as length 11) CAZAC sequence by repeating its first element(s) at the end (cyclic extension), although the resulting sequences do not fulfill the definition of a CAZAC sequence. Alternatively, CAZAC sequences can be generated through a computer search for sequences satisfying the CAZAC properties.
Different cyclic shifts of the same CAZAC sequence provide orthogonal CAZAC sequences. Therefore, different cyclic shifts of the same CAZAC sequence can be allocated to different UEs in the same RB for their RS, ACK/NAK, or CQI transmission and achieve orthogonal UE multiplexing. This principle is illustrated in FIG. 8.
In order for the multiple CAZAC sequences 810, 830, 850, 870 generated correspondingly from multiple cyclic shifts 820, 840, 860, 880 of the same root CAZAC sequence to be orthogonal, the cyclic shift value Δ 890 should exceed the channel propagation delay spread D (including a time uncertainty error and filter spillover effects). If TS is the duration of one symbol, the number of cyclic shifts is equal to the mathematical floor of the ratio TS/D. The cyclic shift granularity equals an element of the CAZAC sequence. For a CAZAC sequence of length 12, the number of possible cyclic shifts is 12 and for symbol duration of about 66 microseconds (14 symbols in a 1 millisecond sub-frame), the time separation of consecutive cyclic shifts is about 5.5 microseconds.
The CQI transmission parameters, such as the transmission RB and the transmission sub-frame, are configured for each UE through higher layer signaling and remain valid over time periods much longer than a sub-frame. Similarly, the ACK/NAK transmission parameters due to persistent PDSCH scheduling and the persistent PUSCH transmission parameters (such as the RB and sub-frame) also remain the same over comparable time periods.
A consequence of SC-FDMA signaling is that the transmission bandwidth of a signal needs to be contiguous. In order to avoid bandwidth fragmentation for PUSCH transmissions, the PUCCH transmissions need to be placed towards the two ends of the operating bandwidth. Otherwise, if there are RBs available on each side of the PUCCH transmission bandwidth, they cannot be used for PUSCH transmission by the same UE while preserving the single carrier property of the transmission.
Moreover, as PUCCH transmission includes periodic CQI transmissions, periodic ACK/NAK transmissions, and dynamic ACK/NAK transmissions, an appropriate ordering for the corresponding RBs at the two ends of the operating bandwidth needs to be determined.
In addition to PUCCH transmission, persistent scheduling of PUSCH transmissions also results in similar bandwidth occupancy characteristics as the PUCCH.